The present application generally relates to communication circuits and systems. More specifically, the present application relates to a receiver or transmitter using image frequency rejection and negative (or positive) frequency rejection in the digital domain.
Radio frequency (RF) reception is often utilized in various mobile communication equipment (e.g., cordless telephones, VHF, UHF, bluetooth, global positioning systems (GPS), computers, handheld computers, satellite radios, etc.). RF reception often requires that an incoming signal (such as a high frequency signal (e.g., 1.5 gigahertz)) be selected with respect to various unwanted other signals.
The incoming signal generally has information encoded upon it and often has a signal level that is extremely low. To obtain the encoded information from the incoming signal, the incoming signal is converted to a lower frequency and its signal level is increased so that a demodulator or an analog-to-digital converter can obtain the encoded information. Accordingly, most conventional receiver architectures process the incoming signal by amplifying the incoming signal to an appropriate level and removing unwanted interfering signals before providing the incoming signal to the demodulator or analog-to-digital converter. This process is preferably done without introducing much distortion. Further, in battery operated devices, all these functions are preferably achieved with minimal power consumption.
In conventional receivers, RF reception is often accomplished by analog super-heterodyne receivers. These receivers down convert the radio frequency (RF) signal to one or more lower, intermediate frequency (IF) signals using analog components. U.S. Pat. No. 5,802,463 discusses conventional super-heterodyne receivers and direct conversion receivers.
The IF signals have fixed, or at least restricted, frequencies which allow the IF signals to be more easily filtered, amplified, and otherwise processed. In conventional super-heterodyne receivers, an antenna provides RF signals which are fed into a band pass RF filter which selectively passes only the RF signals (both desired and otherwise) and noise within a bandwidth of interest while attenuating other RF signals and noise outside this bandwidth, thereby reducing the necessary dynamic range of the succeeding stages. The band-limited RF signals and noise are then amplified by a low noise amplifier (LNA).
Power consumption of the amplifier can be reduced if it is preceded by filters which remove unwanted interference. To assist the RF filter in attenuating electrical noise and signals that are amplified by the LNA and fall within the image frequency band—which are especially critical because they can pass unfiltered through the intermediate frequency (IF) section—the amplified RF signal from the LNA is filtered by an image filter. A mixer mixes the amplified RF signal with a local oscillator (LO) frequency signal to convert the band-limited RF signals to an IF band along with undesired mixing products.
The IF signals from the mixer are generally coupled to an IF filter, which passes mainly the sub-band containing the desired signals. This (and any succeeding) IF filter passes without further attenuation the remnants of any undesired signals and noise present in the image sub-band of the image band which were insufficiently filtered by the RF filter and image filter.
In the process of propagation through the IF filters and amplifiers, the desired IF signal present in the sub-band passed by the filters is selected in favor of signals and noise present at other sub-bands in the IF. The selected IF signal is typically demodulated and translated into a base-band information signal for use by the communications control system. Many variations of the analog super-heterodyne design exist.
One disadvantage of the conventional analog super-heterodyne design is that it cannot easily be fully integrated onto an integrated circuit (IC, or microchip). Most super-heterodyne receivers require significant pre-conversion filters and high quality narrow band IF filters which operate at high frequencies and are comprised of analog components. Accordingly, the analog nature of super-heterodyne receivers results in a larger size, higher cost, and higher power consumption.
The trade-off governing the image-reject filters has motivated designers to seek other methods of suppressing the image. One such method uses a Hartley architecture. An example of a conventional Hartley analog circuit 1000 applied to Direct Conversion Architecture is shown in FIG. 1A. With reference to FIG. 1A, Hartley circuit 1000 mixes an RF input with the quadrature phases of the Local Oscillator (LO), Cos(ωt) and Sin(ωt), low-pass filters the resulting signals and shifts one by 90 degrees before summing the resulting signals together. The Cos(wt) and Sin(wt) signals are at the LO frequency. Circuit 1000 includes analog mixers 1020A-B, an amplifier 1018, analog filters 1022A-B, an analog phase delay circuit 1026, and an analog summer 1028. When the LO frequency is same as RF frequency, the resulting IF is zero.
Since e(jwt)=[Cos(wt)+j Sin (wt)]/2, the process of FIG. 1A can be thought of as multiplying the input by a single positive (or negative) frequency LO. Multiplying in time domain is equivalent to convolution in frequency domain.
In the resulting IF, wanted and image frequencies are separated as positive and negative frequencies. Following Hilbert filter removes the image. Note that shifting by 90 deg. and adding is equivalent to Hilbert filter. On the other hand, if frequency of LO is different from RF frequency then, the wanted and image frequencies are down-converted around an IF. An example waveform for a GSM Low IF scheme is shown in FIG. 1B. A complex band-pass filter (represented by the dashed line in FIG. 1B) then removes the image.
In essence, when the input is real, image is separated by multiplying it with a single complex frequency. Not only image is removed but all the negative (or positive) frequencies are also suppressed; in other words, suppressed image band is very wide.
The same principle can be applied when the input is complex. To realize multiplication of two complex quantities, more operations need to be performed. As shown in FIG. 2, four multiplications (Mixers) and two Summations (since subtraction can be viewed as summation of a negative quantity) are required for a complex multiplication. FIG. 2 implements the following equation: (A+jB)*(C+jD)=(AC−BD)+j(AD+BC). See, “Low IF Topologies for High Performance Analog Front-Ends of Fully Integrated Receivers” IEEE Transactions on Circuits and Systems-II Analog and Digital Signal Processing, Vol. 45, No. 3, March 1998.
It should be noted that, the same concept of image reject architecture is equally valid in a transmit mode. In general, the transmit mode involves an up-convert scheme from an IF. Similar complex multiplication with LO can translate the IF to an RF band.
Subsequently, designers have tried to reduce the analog circuits and do more in digital domain. Some techniques have used sampling mixers and discrete signal processing.
Certain articles have discussed RF sampling mixer architectures that utilize digital filtering and digital base-band down conversion. These articles include: K. Muhammad et al., “A Discrete-time Bluetooth Receiver in a 0.13 um Digital CMOS Process,” Proc., of IEEE Solid-state Circuits Conf., sec. 15.1, pp 268-269, 527, February 2004; K. Muhammad and R. B. Staszewski, “Direct RF Sampling Mixer with Recursive Filtering in Charge Domain,” Proc., of 2004 IEEE Intl. Symp. on Circuits and Systems, sec., ASP-L29.5, May 2004; S. Karvonen et al., “A Low Noise Quadrature Subsampling Mixer,” Proc., of the ISCAS, pp 790-793, 2001. Processing in the digital domain using sampling mixers to down convert the signal followed by a chain of digital filters has been proposed for receiver architectures. Each digital filter is designed to reject as much as possible of unwanted frequencies, without distorting the desired signal. First two listed articles mainly concentrate on using a chain of FIR low-pass filters. The last listed article proposes complex filters after input is down converted to an IF and using sub-sampling techniques.
There is a need for a receiver that is more compatible with digital designs. Further still, there is a need for a receiver that does not utilize a conventional analog design. Further still, there is a need for a digital receiver capable of higher selectivity, sensitivity and fidelity using digital logic devices. Further still, there is a need for a receiver architecture that is low cost, has a small area and has low power consumption when compared to conventional analog-intensive designs. Yet further still, there is a need for a digital receiver optimized for image separation followed by complex filtering.